Flicker-induced Retinal Blood Flow Changes in Diabetes

Diabetic retinopathy is a common long-term complication of diabetes with significant sight-threatening potential. In the recent years much interest was directed towards the investigation of retinal vessel diameters in diabetes, because it has been shown that venous dilatation is an independent risk factor for the incidence of cardiovascular events. Measurement of retinal vessel diameters in these studies is based on fundus photography. We and others have shown that diabetes is not only characterized by retinal venous dilatation but also by a reduced retinal vasodilator response to diffuse luminance flicker occurring early in this disease. Whether this reduced flicker response is an independent risk factor for cardiovascular events is currently under investigation in the longitudinal Maastricht Study using the Dynamic Vessel Analyzer (DVA). Retinal vessel dilatation in response to stimulation with diffuse flicker light occurs due to a phenomenon called neurovascular coupling. This means that increased activity of neurons is associated with a release of endogenous vasodilator substances, which in turns leads to vasodilatation and increased blood flow. In most previous studies only retinal arterial and venous vasodilatation was measured during flicker stimulation. In a study on healthy subjects we, however, showed that there is a pronounced increase in retinal blood velocity during flicker stimulation indicating that most of the vasodilator effects occur in the pre-capillary arterioles which cannot be directly visualized with the DVA due to limited resolution. In this previous study we employed a laser Doppler velocimeter (LDV) for the measurement of retinal blood velocities. This technique does, however, require long measurement times, sufficient operator experience and significant compliance from the subjects under study, which prevents usage in clinical routine or large scale studies. In the recent years we have developed an alternative system for the measurement of retinal blood velocities. This technique is called bi-directional Fourier Domain Optical Doppler Tomography. This technique combines the principles of LDV with those of Optical Coherence Tomography. In present grant proposal we propose to combine this technique with the DVA. This will allow for the real time measurement of retinal blood flow during diffuse luminance flicker. As such the system should enable to directly assess retinal blood flow in response to diffuse luminance flicker in vivo. In addition to studies in healthy volunteers studies in patients with diabetes are scheduled to test the hypothesis that the retinal blood flow response to diffuse luminance flicker is reduced in patients with diabetes. This hypothesis is based on our previous results showing that flicker-induced vasodilatation is reduced in diabetes.

In the present project it was intended to measure neurovascular coupling in the retina using Doppler optical coherence tomography OCT. For this purpose the retina was stimulated with diffuse luminance flicker and retinal blood flow was measured concomitantly. As proposed in the grant application we coupled a system for visual stimulation of the retina to an existing Doppler OCT system. In addition, the function of the retina was investigated using multifocal electroretinography and the retinal nerve fiber layer was measured using OCT. Our results show that in patients with early diabetes that do not show any alterations in the retina neurovascular coupling is already abnormal. Alterations in neurovascular coupling appear therefore to be the earliest changes that can be seen in type 1 diabetes. The neurovascular unit is therefore an attractive target for therapeutic interventions in diabetic retinopathy.